1. Field of the Invention
The present invention relates to an electrochemical element such as a battery or an electric double layer capacitor having a prismatic outer shape and comprising an electrode assembly composed of positive and negative electrodes wound into a flat shape, and being accommodated into a case together with an electrolyte. In particular the invention relates to an electrochemical element having a current collection structure designed for high power output.
2. Description of the Related Art
In an electrochemical element such as a battery or an electric double layer capacitor, an electrode assembly having a winding structure composed of positive and negative electrodes wound with a separator interposed therebetween is widely employed in order to increase a reaction area per unit volume. The electrode assembly is accommodated in a metal case with an electrolyte, and the case is sealed. An electrochemical element is configured through connecting the positive and negative electrodes to portions serving as positive and external negative connection terminals, respectively, for allowing an external connection.
The positive and negative electrodes are formed by coating an active material to a current collector made of a foil or a thin metal mesh plate, and thus a current path is created through connecting the positive and negative current collectors to portions serving as positive and external negative connection terminals, respectively, by leads. In this case, when only one portion of the current collector is connected to the portion serving as an external connection terminal, a current path from a distant portion in the wound strip-shaped current collector to the connected portion becomes longer, resulting in an increase in electrical loss and temperature rise during a high-rate charge/discharge. Particularly, in the case of an electric double layer capacitor in which energy can be charged/discharged in a short period of time, to advantageously utilize this property, the internal resistance of a current path should be reduced as much as possible.
In order to solve the above problems in the connection between an electrode assembly and external connecting portions for positive and negative electrodes, a battery structure has been developed for improving current collection efficiency (see Japanese Patent Laid-Open Publication No. 2000-294222). In this case, an electrode assembly is formed into a cylindrical shape through winding positive and negative electrodes with a separator interposed therebetween such that part of a current collector of the positive or negative electrode is protruded from one end of the electrode assembly. The current collector protruded from the end is bent inward by pressing the protruded collector in a direction of cylinder axis to form flat regions, and a current collecting plate is welded to the flat regions.
Another battery structure which is suitable for assembling a high power battery pack for an electric vehicle or the like has been developed (see Japanese Patent Laid-Open Publication No. 2000-040501). In this structure, strip-shaped positive and negative electrodes each having a current collector exposed from an edge in the width direction of the strip are wound around a flat winding core to which metal plates serving as positive and negative terminals are attached. The positive and negative terminals are welded to the exposed current collector portions of the positive and negative electrodes, respectively, to form a flat wound electrode, and the end of the each of the positive and negative terminals is extended from a battery case.
Besides the above two examples for a battery, a configuration for an electrical double layer capacitor formed through winding positive and negative electrodes has been developed (see Japanese Patent Laid-Open Publication No. Hei 10-050556). In this case, an electrode assembly is formed such that a plurality of current collecting leads are projected from the current collectors of the wound positive and negative electrodes along a line in a radial direction. The plurality of current collecting leads arranged in the radial direction are bundled, and the bundled portion is clamped with a terminal member made of the same material as the current collecting lead. The clamped portion is then welded to form an output terminal.
However, in the configuration disclosed in Japanese Patent Laid-Open Publication No. 2000-294222 in which a flat surface is formed through bending a current collector protruded from an end and welded with a current collector plate placed thereon, the welding conditions cannot be stabilized when a thin electrode current collector is employed for obtaining high capacity, resulting in a frequent short circuit due to heat fusion of a separator having a poor heat resistance. Particularly, this phenomenon often occurs in a lithium ion battery employing a thin metal foil serving as a current collector, resulting in the reduction in production yield.
The configuration disclosed in Japanese Patent Laid-Open Publication No. 2000-040501 in which positive and negative electrodes are wound around a winding core into a flat shape can be advantageously applied to an apparatus having a relatively large installation space such as an electric vehicle. However, since the winding core is required, the thickness of the battery cannot be reduced as a flat prismatic battery. Therefore, the battery has a disadvantage when the battery is employed in an apparatus requiring downsizing or a plurality of the batteries are integrated to form a battery pack.
In addition, in the configuration disclosed in Japanese Patent Laid-Open Publication No. Hei 10-050556 in which a plurality of current collecting tabs are formed in the radial direction of cylindrically wound positive and negative electrodes, the plurality of current collecting tabs are protruded from the electrodes which are cut into a strip shape. Therefore, the electrodes cannot be cut in an efficient manner, resulting in poor productivity.